The present invention relates generally to the field of fossil-fueled boilers and steam generators used in the production of steam for industrial processes or utility power generation. More particularly, the present invention relates to an internal flue gas by-pass damper which can be used to control the temperature of the flue gas provided to downstream devices by controlling the effectiveness (ability to absorb heat) of the final banks of heat exchanger surfaces forming a part of the boiler or steam generator.
Nitrogen oxides (NOx), along with sulfur oxides (SOx) and particulate matter, are one of the primary pollutants emitted during combustion processes, and are the subject of various state and national regulatory policies. For a general description of these environmental considerations and equipment and methods used to control these pollutants, the reader is referred to STEAM/its generation and use, 40th Edition, Stultz and Kitto, Jr., Eds., Copyright (copyright) 1992, The Babcock and Wilcox Company, particularly Chapters 32 through 36.
As described in Chapter 34 of STEAM 40th, pages 34-1 through 34-9, the text of which is hereby incorporated by reference as though fully set forth herein, one method for controlling NOx emissions produced by such boilers or steam generators comprises systems employing selective catalytic reduction (SCR) technology. SCR systems catalytically reduce flue gas NOx to N2 and H2O using ammonia in a chemical reaction. The NOx reduction reactions take place as the flue gas, into which ammonia has been introduced and mixed, passes through a catalyst chamber.
The proper operation of such SCR systems requires that the temperature of the flue gas entering the SCR system be controlled within a specific temperature range, typically 450 to 840 F. (232 to 449 C.). Often, optimum performance occurs between 675 and 840 F. (357 and 449 C.), and it is desirable to maintain the temperature of flue gases leaving the last heat exchanger of a boiler and entering an SCR at or above the NOx reduction catalyst""s minimum operating temperature. However, the flue gas temperature leaving a boiler or steam generator does not remain constant with changes in boiler or steam generator load (as defined and measured by the amount of steam production), the flue gas temperature exiting from the boiler or steam generator typically decreasing as steam production decreases. Nonetheless, the minimum flue gas temperature entering the catalyst must be maintained at or above the NOx reduction catalyst""s minimum operating temperature even as the boiler load is reduced.
The current method of keeping the temperature of the flue gas leaving the last heat exchanger surface of a boiler or steam generator at or above the SCR NOx reduction catalyst""s minimum operating temperature as the boiler load is reduced is to install a by-pass flue system. The by-pass flue diverts and transports a portion of the flue gases from a point upstream of the last heat exchanger surface in the boiler or steam generator (which is typically a bank of economizer heat transfer surface) to a location just upstream of the SCR. The diverted flue gases flowing through the by-pass flue is at a higher temperature since they did not pass over (and transfer heat to) the economizer heat exchanger surfaces. Thus, the diverted flue gases raise the overall temperature of the flue gases entering the SCR above the minimum temperature requirement.
In certain cases where existing boilers or steam generators are being retrofitted with SCR systems there is either insufficient space to install such a by-pass flue, or to install such a by-pass flue in a cost-effective manner. Some situations may require major building modifications, relocation of major equipment, and/or significant boiler pressure part changes. In such cases, there would be either no control over the temperature of the flue gases entering the SCR, or the installation of a by-pass flue would be so costly as to be prohibitive of making the modification. A system which would permit retrofit installation of such SCR systems in situations where provision of such flue gas by-pass flues is impractical, while still providing control of the flue gas temperature entering the SCR, would be welcomed by industry.
The present invention can be used to control the temperature of flue gas produced by a boiler or steam generator based upon the temperature requirements of a downstream (with respect to a direction of flue gas flow) device which receives the flue gas. A particular application of the present invention involves an apparatus for controlling the flue gas temperature entering a downstream selective catalytic reduction (SCR) system used to reduce atmospheric NOx emissions from the boiler or steam generator.
One aspect of the present invention is thus to provide a device for controlling the temperature of flue gases entering an SCR which does not require significant additional space and modifications typically required when flue gas by-pass flues are added outside of the existing flues associated with the boiler or steam generator.
Another aspect of the present invention is to provide a device for maintaining the temperature of flue gas provided to an SCR system above an SCR minimum operating temperature at different boiler load conditions.
In its most basic form, the present invention dispenses with the need to provide the known flue gas by-pass flue systems and instead adjusts the heat transfer effectiveness (ability to absorb heat) of the final banks of heat exchanger surface (typically economizer and primary superheater surface) of the boiler or steam generator to provide a variable effectiveness heat transfer surface for controlling the temperature of the flue gases entering the SCR.
Accordingly, one aspect of the present invention is drawn to a flue gas passage arrangement for a steam generator which permits adjustment of the heat transfer effectiveness of a final bank of heat exchanger surface to control a temperature of the flue gas flowing through the flue gas passage and conveyed to a downstream NOx reduction device having a minimum operating temperature. Economizer heating surface is located within the flue gas passage. A baffle plate extends through the flue gas passage and creates two flue gas paths there through. The economizer heating surface is located in at least one of a first and a second section defined by the flue gas paths, one section in each flue gas path. Finally, damper means are provided, positioned in the second section and connected between the baffle plate and walls of the flue gas passage. The damper means is used for selectively permitting or restricting the flow of flue gases through the second section. In this way, the heat transfer effectiveness of the economizer heating surface is controllable with the damper means to maintain the temperature of the flue gas conveyed to the downstream NOx reduction device at or above the minimum operating temperature of the NOx reduction device.
The variable position damper can be full open to allow flue gases to pass equally over the combined heat exchanger surface area of both sections, or progressively closed, to reduce the effective heat exchanger surface area. The total heat exchanger surface area and apportionment of the total convection pass gas flow area between the two sections is determined based on the maximum and minimum boiler loads and required minimum operating temperature for flue gases entering the NOx reduction device.
Alternatively, two separate heat exchanger banks may be provided divided by a separator plate and one heat exchanger bank has a variable position damper located beneath it.
Still further, another arrangement involves providing the economizer heat exchanger in only one path, with an open path created along one side of the heat exchanger bank. A variable position damper is provided at the end of the open path for variably restricting flue gas flow through that path.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific benefits attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.